I also noticed that price of $21.99. I believe this is some sort of anomaly. I called Amazon to inquire about the price because I ordered the first two volumes from them, and I have pre-ordered volume 3 from them. The customer service rep wasn't able to give me a specific reason why, she only gave some general factors as to what determines the price of an item. She did confirm what I already knew, that if you pre-order the item and the price drops between the time you order the item and the time the order ships, you get the lowest advertised price.
Unless Sony has raised the price of these, we should expect to see the price drop to $16.99 for a short time, then go up to the usual $19.99. This is probably the case because the MSRP has not changed for volume 4 according to Amazon's listing. Volume 4's MSRP is still $24.96, the same as the first 3 volumes.